1. Field of the Invention
The present invention relates generally to cathode ray tube (CRT) bulbs. More specifically, the present invention relates to CRT bulbs having flat front panels, or faceplates, suitable for use with flat tensioned shadow masks.
2. Discussion of the Related Art
As is known the art of CRT construction, a CRT "bulb" is formed from a screen-bearing front glass panel affixed at high temperatures to a glass funnel section with cementitious material, normally a devitrifiable solder glass, or "frit". A CRT "envelope" is then formed by sealing an electron gun into a neck section of the bulb opposite the screen. The CRT envelope is then evacuated and sealed to become an operational, or finished, CRT.
Because the CRT is evacuated, atmospheric pressure produces stress on the CRT envelope. Thus the CRT must be designed so that the weakest portion of its envelope is able to withstand this atmospheric loading. This ability to withstand atmospheric loading is referred to as the "pressure strength" of the bulb. Pressure strength is determined by a defacto industry standard test. Even though the external pressure after evacuation is approximately 14.7 psi; new bulbs are required to have a pressure strength greater than 14.7 psi in order to insure that they can survive a lifetime of environmentally induced damage The funnel-to-panel seal area, hereinafter "seal area", encompassing the funnel walls, frit, and panel areas adjacent each other, is a highly stressed area and therefore more likely to be an origin of failure; largely because large bending moments are typically generated in this area due to panel deflection.
In the common CRT spherical faceplate, the faceplate, being analogous to an arch, has a shape which inherently resists the atmospheric load on the CRT. However, in the case of a tensioned foil shadow mask CRT, which most commonly uses a flat front panel, the flat front panel shape does not inherently resist the atmospheric loading as well as a spherical panel.
As seen in FIG. 1, a known type of flat tension mask (FTM) CRT has a flat, skirtless, glass front panel 11 sealed to a glass funnel 13. The glass front panel 11 has a mask support structure 14 affixed on its interior surface 16. The mask support structure 14 surrounds a luminescent screen 18. Affixed under tension to the mask support structure 14 is a foil shadow mask 20.
As seen in FIG. 2, a flat skirtless glass front panel 11 is bonded to a glass funnel 13 by means of a devitrifying frit 15. The devitrifying frit 15 is applied to either the funnel 13 or front panel 11 at room temperature in the form of a paste. The front panel and funnel are then suitably fixtured together and traversed through a lehr, or oven, gradually bringing the temperature of the funnel and panel, i.e. the inchoate bulb, up to approximately 440.degree. C. for a period of time sufficient to devitrify the frit. See FIG. 3 for a frit sealing cycle utilized for a common devitrifying frit denominated CV-685 and made by O-I/NEG Co. During sealing, the frit becomes rigid and crystalline at its "setting temperature", here 440.degree. C., and remains so thereafter unless much higher temperatures are applied to it, thus sealing the funnel to the panel and forming a CRT bulb. The temperature of the bulb is then gradually reduced from the setting temperature back down to room temperature, 25.degree. C., in order for further processing to take place.
As noted above, in conventional tube processing the coefficients of thermal expansion (CTE) are deliberately matched. In this case, as seen in FIG. 1, the CTE of the funnel 13 is represented in FIG. 1 by arrow 17 and the CTE of the faceplate or front panel 11 is represented by arrow 19.
Upon cooling of the bulb of FIG. 2, the residual stress in the bulb results in a neutral N, or minimally stressed, seal area 12. As seen in FIG. 4, upon evacuation, atmospheric pressure strains the seal area 12, which results in the funnel 13 being placed in tension, T, on the outside of the CRT and in compression, C, on the inside of the CRT. The area of the CRT placed in tension, i.e. the outside of the funnel 13 near the funnel-to-panel frit seal, is subject to environmental damage, e.g., scratches, bumps, etc. The flaws associated with this damage, because they are under tension and exposed to atmospheric moisture, will propagate over time. This is a well known phenomenon known as stress corrosion. The rate of flaw propagation is a function of the tensile stress at the flaw. If the stress level is high enough, the bulb may fail prematurely.
As graphically represented in FIGS. 2, 4 & 5 the vulnerable funnel area starts at neutral residual stress in the unevacuated envelope and upon evacuation due to panel-inflection-induced strain undergoes a stress change to place the vulnerable funnel area in tension.
In standard construction of the aforementioned flat front panel CRT, the flat front panel is connected by frit to a funnel to form a rigid bulb. Common practice in the art dictates that the coefficients of thermal expansion (CTEs) of the funnel and panel be closely matched so as to not create residual stresses caused by CTE mismatch in the bulb when it is cooled to room temperature. But, upon evacuation of the envelope, as the front panel deflects, large bending stresses will be placed on the frontal seal land area creating tension on the funnel and a potential source of CRT failure. In order to minimize the panel deflection and deflection-induced seal area stress, standard flat front panel CRT construction utilizes a thick glass for a stiffer front panel and a thick funnel seal land. The thick front panel will reduce deflection induced strains in the seal area and the thickened front seal land is incorporated into the funnel to further resist the remaining strain. However, in this arrangement, the thick front panel, being designed primarily for stiffness, is stressed well below its allowable limits and therefore represents wasted material in terms of envelope strength. The thickened bulb members add weight and attendant material, and increased panel and/or funnel manufacturing and CRT processing time and shipping costs to the CRT.
The problem of increasing panel weight/thickness has led the inventor to re-think the emphasis on residual stresses formed in the bulb as a result of funnel-to-panel sealing. Per the above discussion, in some cases it would be desirable to redistribute stress in the seal area of an evacuated CRT bulb to favor a thinner paneled bulb With strength equal to a bulb with a thicker panel.